The present invention relates to an exhaust gas purifying apparatus and method for a hybrid car, which are suited for an electric vehicle or the like equipped with an electric motor for driving the vehicle and an internal-combustion engine for generating electric power.
In recent years, the emission regulation on internal-combustion engines is becoming stringent primarily because of the environmental problems, and many new technologies are being researched and developed to comply with the regulation. One of such efforts is represented by the "electric vehicle."
The electric vehicle is driven by an electric motor, and the electric motor basically receives electric power from a battery. The battery, however, has a limited capacity, making it difficult to achieve a long-distance drive. For this reason, the "hybrid car," which is equipped with a power generator for charging the battery and an internal-combustion engine for driving the power generator in addition to the above-mentioned electric motor and battery, is currently regarded as a typical form of the electric vehicle.
However, the hybrid car is, after all, provided with an internal-combustion engine; therefore, how to achieve the exhaust gas purification of the internal-combustion engine is extremely important to fulfill the original object of the electric vehicle to provide a pollution-free automobile. Accordingly, the internal-combustion engine mounted on the hybrid car is adapted for steady driving at, for example, a constant rotational speed, as a rule, to thereby minimize the generation of harmful substances such as CO and NOx by avoiding unsteady driving. Also, an exhaust pipe is provided with a catalyst for purifying exhaust gas as in a general internal-combustion engine, or stoichiometric feedback by an O.sub.2 sensor, lean feedback by an air-fuel ratio sensor, or the like is conducted to purify exhaust gas.
Meanwhile, as a device for powerfully controlling the exhaust gas of an internal-combustion engine, there has been proposed the "heater catalyst," the catalytic action thereof has been dramatically improved by heating the catalyst provided in an exhaust pipe by a heater to activate it.
Nevertheless, although various corrective measures have been taken for purifying the exhaust gas of the conventional hybrid cars as described above, there is still a problem that these measures alone cannot fully reduce the generation of harmful substances in exhaust gas to a level which is required of the electric vehicle.
Meanwhile, the heater catalyst, which is commonly recommended for the conventional internal-combustion engines, cannot provide powerful catalytic action until the catalyst is fully heated and activated; therefore, emission of harmful substances during that period is more or less unavoidable. Thus, to obtain powerful catalytic action from the start of an engine, the catalyst must be preheated before the engine is started so that the activation of the catalyst will have been finished when the engine is started. This is an extremely complicated operation for a driver, adversely affecting the practicality.